Generics with Array List and Hash Set generic

Generics with Array. List and Hash. Set ge·ner·ic adjective jə ˈnerik, -rēk relating or applied to or descriptive of all members of a genus, species, class, or group: common to or characteristic of a whole group or class: typifying or subsuming: not specific or individual. From Wikipedia: generic programming: a style of computer programming in which algorithms are written in terms of to-bespecified-later types that are then instantiated when needed for specific types provided as parameters. In Java: Without generics, every Vector object contains a list of elements of class Object. Clumsy With generics, we can have a Vector of Strings, a Vector of Integers, a Vector of Genes. Simplifies programming, guards 1 against some errors

Generics and Java’s Collection Classes ge·ner·ic adjective jə ˈnerik, -rēk relating or applied to or descriptive of all members of a genus, species, class, or group: common to or characteristic of a whole group or class: typifying or subsuming: not specific or individual. From Wikipedia: generic programming: a style of computer programming in which algorithms are written in terms of to-bespecified-later types that are then instantiated when needed for specific types provided as parameters. In Java: Without generics, every Vector object contains a list of elements of class Object. Clumsy With generics, we can have a Vector of Strings, a Vector of Integers, a Vector of Genes. Simplifies programming, guards 2 against some errors

Generics with Array. List and Hash. Set Array. List v= new Array. List (); An object of class Array. List contains a growable/shrinkable list of elements (of class Object). You can get the size of the list, add an object at the end, remove the last element, get element i, etc. More methods exist! Look at them! v Array. List@x 1 Vector defined in package java. util Array. List@x 1 Object Array. List Fields that contain a list of objects (o 0, o 1, …, osize()-1) Array. List () add(Object) get(int) size() remove(…) set(int, Object) 3 …

Generics with Array. List and Hash. Set s= new Hash. Set(); An object of class Hash. Set contains a growable/shrinkable set of elements (of class Object). You can get the size of the set, add an object to the set, remove an object, etc. More methods exist! Look at them! s Hash. Set@y 2 Hash. Set Don’t ask what “hash” means. Just know that a Hash Set object maintains a set Hash. Set@y 2 Object Hashset Fields that contain a setof objects {o 0, o 1, …, osize()-1} Hash. Set() add(Object) contains(Object) size() remove(Object) 4 …

Iterating over a Hash. Set or Array. List Hash. Set s= new Hash. Set(); Hash. Set@y 2 … code to store values in the set … for (Object e : s) { System. out. println(c); } A loop whose body is executed once with e being each element of the set. Don’t know order in which set elements processed Use same sort of loop to process elements of an Array. List in the order in which they are in the Array. List. Object Fields that Hash. Set contain a setof objects {o 0, o 1, …, osize()-1} Hash. Set() add(Object) contains(Object) size() remove(Object) … s Hash. Set@y 2 5 Hash. Set

Array. List to maintain list of Strings is cumbersome Array. List v= new Array. List (); … Store a bunch of Strings in v … —Only Strings, nothing else // Get element 0, store its size in n String ob= ((String) v. get(0)). length(); int n= ob. size(); All elements of v are of type Object. So, to get the size of element 0, you first have to cast it to String. Make mistake, put an Integer in v? May not catch error for some time. v Array. List@x 1 Array. List @x 1 Object Array. List Fields that contain a list of objects (o 0, o 1, …, osize()-1) Vector() get(int) remove() … add(Object) size() set(int, Object) 6

Generics: say we want Vector of Array. List only API specs: Array. List declared like this: public class Array. List <E> extends Abstract. List<E> implements List<E> … { … } Means: Can create Vector specialized to certain class of objects: Array. List <String> vs= new Array. List <String>(); //only Strings Array. List <Integer> vi= new Array. List <Integer>(); //only Integers vs. add(3); vi. add(“abc”); These are illegal int n= vs. get(0). size(); vs. get(0) has type String No need to cast 7

Generics allow us to say we want Vector of Strings only API specs: Vector declared like this: public class Vector<E> extends Abstract. List<E> implements List<E> … { … } Full understanding of generics is not given in this recitation. E. g. We do not show you how to write a generic class. Important point: When you want to use a class that is defined like Vector above, you can write Vector<C> v= new Vector<C>(…); to have v contain a Vector object whose elements HAVE to be of class C, and when retrieving an element from v, its class is C. 8

Package java. util has a bunch of classes called e Collection Classes that make it easy to maintain sets of values, list of values, queues, and so on. You should spend dome time looking at their API specifications and getting familiar with them. Interface Collection: abstract methods for dealing with a group of objects (e. g. sets, lists) Abstract class Abstract. Collection: overrides some abstract methods with real methods to make it easier to fully implement Collection 9

Interface Collection: abstract methods for dealing with a group of objects (e. g. sets, lists) Abstract class Abstract. Collection: overrides some abstract methods with methods to make it easier to fully implement Collection Abstract. List, Abstract. Queue, Abstract. Set, Abstract. Deque overrides some abstract methods of Abstract. Collection with real methods to make it easier to fully implement lists, queues, set, and deques Next slide contains classes that you should become familiar with and use. Spend time looking at their specifications. There also other useful Collection classes 10

Array. List extends Abstract. List: An object is a growable/shrinkable list of values implemented in an array Hash. Set extends Abstract. Set: An object maintains a growable/shrinkable set of values using a technique called hashing. We will learn about hashing later. Linked. List extends Abstract. Sequential. List: An object maintains a list as a doubly linked list Vector extends Abstract. List: An object is a growable/shrinkable list of values implemented in an array. An old class from early Java Stack extends Vector: An object maintains LIFO (last-infirst-out) stack of objects Arrays: Has lots of static methods for dealing with arrays —searching, sorting, copying, etc. 11

Interface Collection: abstract methods for dealing with a group of objects (e. g. sets, lists) Abstract class Abstract. Collection: overrides some abstract methods with real methods to make it easier to fully implement Collection Array. List implements 3 other interfaces, not shown Iterable Not discussed today Iterable Collection Object Iterable Collection Abstract. Collection List Abstract. List Iterable Collection List Array. List Format of Array. List object 12

Interface List: abstract methods for dealing with a list of objects (o 0, …, on-1). Examples: arrays, Vectors Abstract class Abstract. List: overrides some abstract methods with real methods to make it easier to fully implement List Homework: Object Look at API specifications Abstract. Collection and build diagram giving format of Abstract. List Hash. Set Vector Collection Iterable Not discussed today Iterable Collection List Format of Array. List object 13

Parsing Arithmetic Expressions Introduced in lecture briefly, to show use of grammars and recursion. Done more thoroughly and carefully here. We show you a real grammar for arithmetic expressions with integer operands; operations +, -, *, /; and parentheses ( ). It gives precedence to multiplicative operations. We write a recursive descent parser for the grammar and have it generate instructions for a stack machine (explained later). You learn about infix, postfix, and prefix expressions. Historical note: Gries wrote the first text on compiler writing, in 1971. It was the first text written/printed on computer, using a simple formatting application. It was typed on punch cards. You can see the cards in the Stanford museum; visit 14 infolab. stanford. edu/pub/voy/museum/pictures/display/floor 5. htm

Parsing Arithmetic Expressions -5 + 6 Arithmetic expr in infix notation 5 – 6 + Same expr in postfix notation infix: operation between operands postfix: operation after operands prefix: operation before operands PUSH 5 NEG PUSH 6 ADD Corresponding machine language for a “stack machine”: PUSH: push value on stack NEG: negate the value on top of stack ADD: Remove top 2 stack elements, push their 15 sum onto stack

Infix requires parentheses. Postfix doesn’t (5 + 6) * (4 – 3) Infix 5 6 + 4 3 - * Postfix 5+6*3 5 6 3 * + Infix Postfix Math convention: * has precedence over +. This convention removes need for many parentheses Task: Write a parser for conventional arithmetic expressions whose operands are ints. 1. Need a grammar for expressions, which defines legal arith exps, giving precedence to * / over + 2. Write recursive procedures, based on grammar, to parse the expression given in a String. Called a recursive descent parser 16

Use 3 syntactic categories: <Exp>, <Term>, <Factor> Grammar A <Factor> has one of 3 forms: 1. integer 2. – <Factor> 3. ( <Exp> ) <Factor> : : = int | <Factor> | ( <Exp> ) <Factor> – <Factor> 3 – <Factor> – Show “syntax trees” for 3 –– 5 –(3+2) 5 ( <Exp> ) 3 + 2 Haven’t shown <Exp> grammar 17 yet

Use 3 syntactic categories: <Exp>, <Term>, <Factor> Grammar A <Term> is: <Factor> followed by 0 or more occurs. of multop <Factor> where multop is * or / Means: 0 or 1 occurrences of * or / <Term> : : = <Factor> { {* | /}1 <Factor> } <Term> <Factor> Means: 0 or more occurrences of thing inside { } <Exp> 3 * ( 5 +2 ) * 6 18

Use 3 syntactic categories: <Exp>, <Term>, <Factor> Grammar A <Exp> is: <Term> followed by 0 or more occurrences of addop <Term> where addop is + or <Exp> : : = <Term> { {+ | -}1 <Term> } <Exp> <Term> <Factor> 3 + ( <Term> <Factor> 5 + 2 ) _ 6 19

Class Scanner Initialized to a String that contains an arithmetic expression. Delivers the tokens in the String, one at a time Expression: 3445*(20 + 16) Tokens: 3445 * ( 20 + 16 ) All parsers use a scanner, so they do not have to deal with the input character by character and do not have to deal with whitespace 20

An instance provides tokens from a string, one at a time. A token is either Class Scanner 1. an unsigned integer, 2. a Java identifier 3. an operator + - * / % 4. a paren of some sort: ( ) [ ] { } 5. any seq of non-whitespace chars not included in 1. . 4. public Scanner(String s) public boolean has. Token() public String token() public String scan. Over. Token() public boolean token. Is. Int() public boolean token. Is. Id() // An instance with input s // true iff there is a token in input // first token in input (null if none) // remove first token from input // and return it (null if none) // true iff first token in input is int // true iff first token in input is a 21 // Java identifier

/** scanner's input should start with a <Factor> Parser for —if not, throw a Runtime. Exception. <Factor> Return the postfix instructions for <Factor> and have scanner remove the <Factor> from its input. <Factor> : : = an integer | – <Factor> | ( <Expr> ) */ public static String parse. Factor(Scanner scanner) The spec of every parser method for a grammatical entry is similar. It states 1. What is in the scanner when paring method is called 2. What the method returns. 3. What was removed from the scanner during parsing. 22

/** scanner's input should start with an <Exp> Parser for --if not throw a Runtime. Exception. <Exp> Return corresponding postfix instructions and have scanner remove the <Exp> from its input. <Exp> : = <Term> { {+ or -}1 <Term>} */ public static String parse. Exp(Scanner scanner) { String code= parse. Term(scanner); while ("+". equals(scanner. token()) || "-". equals(scanner. token())) { String op= scanner. scan. Over. Token(); String right. Op= parse. Term(scanner); code= code + right. Op + (op. equals("+") ? "PLUSn" : "MINUSn"); } return code; 23 }